This invention relates to the control of heat exchangers and, more particularly, to a method of balancing the temperature of a plurality of heat exchangers connected in parallel relationship and each having a feed gas stream and a stream of a heat medium such as return gas which are alternately switched over at certain intervals.
In general, it has been a common practice to employ reversing heat exchangers in an air separation plant, whose performance efficiency will vary in dependence on temperature variations among a plurality of heat exchangers. The reversing heat exchanger usually includes a feed air stream, a return gas stream comprising impure nitrogen and oxygen gases, and a separated gas stream comprising product nitrogen and product oxygen. The feed air stream and the return gas stream are alternately and periodically switched over by the control of switching valves, whereas separated gas flows through a fixed heat exchange channel. During the heat exchange process, since the feed air is cooled while passing through a feed air stream channel, impurities contained in the feed air often adhere to the wall of the stream channel in the form of ice or frozen CO.sub.2. These can accumulate within the heat exchanger, resulting in unstable operation of the plant. In extreme cases, it becomes impossible to continue the operation of the heat exchangers. It is to solve this problem that the feed air stream and the return gas stream are alternately changed over, thereby preventing the accumulation of such impurities within the heat exchangers during normal operation. By ths expedient, the accumulation of impurities and dry ice are satisfactorily avoided by the vapor pressure difference between such impurities contained in the feed air stream and return gas stream. Since the amount of feed air being supplied to a recitification column will temporarily be reduced for a brief interval during switching operation of the feed air stream and the return gas stream, it is desirable not to switch the streams of all of the heat exchangers at one time but rather to switch them over sequentially, with a fixed time difference between each operation. In order to remove the impurities from the walls of the stream channel in a more effective fashion, it is desirable that the temperature difference between the feed air stream and the return gas stream be maintained within a given acceptable range. In actual practice, however, the temperature difference between the feed air stream and the return gas stream in the heat exchanger is such that the temperature difference is small at the feed air stream inlet end of the heat exchanger and increases towards the opposite end. If this temperature difference exceeds a certain level such that it is intersected by the boundary curve for the evaporation of frozen CO.sub.2, removal of the frozen CO.sub.2 becomes impossible. This phenomenon is caused by the fact that the specific heat at constant pressure for the low pressure return gas, varies over a limited small range, whereas the specific heat at constant pressure for the high pressure feed air, becomes greater as the temperature decreases. To overcome this problem, it has been a usual practice to provide a reverse flow line for directing gases derived from the rectification column in the reverse direction to the flow of the feed air toward the hot end of the heat exchanger, thereby controlling the temperature difference between the feed air stream and the return gas stream within an acceptable range.
Normally, the reversible heat exchanger of the kind described above has a size of 1200 .times. 1200 mm in cross section which has been a maximum size in industrial plants from manufacturing considerations. In conventional larger air separation plants, a number of heat exchangers of the above-mentioned size are usually connected in parallel. In these plants, it has been a common practice to monitor a set of thermometers mounted at the middle and the cold end of each heat exchanger, to assist in controlling the flows of feed air and return gas by manually operated flow control valves. This is done so as to maintain the temperatures at the middle and the cold end of each heat exchanger within acceptable ranges. Another expedient proposed in the prior art is to provide an automatic regulator valve at the inlet of the feed air channel of each heat exchanger and provide temperature control device having temperature sensing elements disposed at the outlet of the feed air channel, adapted to be controlled in cascade to provide automatic control. In recently employed large air separation plants, however, twenty or more heat exchanger cores are installed, and the control of individual cores will interfere with each others, resulting in inefficient air separation and hunting. Consequently, disturbances will take place in temperature distribution within the heat exchangers resulting in the production of product gases of low purity and in low heat exchange efficiency.
To solve the above problems, it has therefore been proposed to provide a total control system for a number of heat exchangers instead of controlling the temperature distribution of individual heat exchangers. The total control system is, for example, disclosed in U.S. Pat. No. 3,167,113 entitled "Equalization of Loads on Heat Exchangers" by Louis D. Kleiss. This prior art is concerned with a heat exchange system, not a reversing heat exchanger system, including a plurality of heat exchangers connected in parallel relationship. This prior art system features the provision of temperature sensing elements in a gas feed conduit upstream and downstream from a heat exchanger and additional temperature sensing elements disposed in a product gas conduit upstream of the heat exchanger, to detect temperatures Tx, Ty and Tz. These temperatures can be used to express the performance of the heat exchanger as follows: EQU Performance = (Tx - Ty) / (Tx - Tz)
In this manner, the performance of each heat exchange is calculated, thereby providing an average performance for all of the heat exchangers, which is utilized for controlling valves disposed in the gas feed conduits upstream of the respective heat exchangers thereby equalizing the performance of all the heat exchangers. In another embodiment of this prior patent, temperature sensing elements are disposed in the heat exchanger to sense the temperature difference between hot and cold fluids to provide an output representative of the average temperature difference in the heat exchanger. This output is supplied to a controller which actuates flow control valves disposed in the gas feed conduits upstream of the heat exchangers, to thereby equalize the temperature difference between the hot and cold fluids in each heat exchanger. In this prior art, the performance of each heat exchanger or the temperature difference between the hot and cold fluids are utilized for controlling the flow control valves of the respective heat exchangers without regard to the change-over period of the heat exchangers. Consequently, if the principal concept disclosed in this prior art is applied to a heat exchanger system of the reversing heat exchanger type with a view to equalizing the temperature difference between the hot and cold fluids in each heat exchanger, each individual heat exchanger will be controlled irrespective of the trend in temperature variation of all of the heat exchangers and, therefore, controlled operations of the respective heat exchangers will be interferred with each other causing hunting phenomenon. As a result, a complete thermal balance between all heat exchangers is not obtained, thereby possibly causing disturbances in the temperature distribution in a rectification column so that the purity of product gas is decreased and performance efficiency of the rectification column is lowered as described hereinabove with respect to the control of individual heat exchangers. Another drawback encountered with the prior art resides in that the manufacturing cost of a heat exchanger system is high because of a number of temperature sensing elements disposed in various parts of the heat exchangers. Further, if the temperature difference between hot and cold fluids is detected only at the middle of a heat exchanger, it has a lower value than the average value of the temperature difference between the hot and cold fluids, and has no relation to variations in temperature distribution within the heat exchangers. It is thus concluded that the above method is not suitable for controlling the temperature balance of a number of reversing heat exchangers.